Random crashes or freezing

Hi,

My computer sometimes crashes or freezes. More often crashes shwoing a black screen with lots of small rectangles which look like they might be dialogue boxes but are very hard to see, as they're so small. It also occasionally freezes, the track pad doesn't allow you to click down. I hold down the power button to reset it, sometimes this can take multiple attempts. The speakers dont work either if that makes a difference, they failed just outside of the apple warranty. I did try new speakers but it made no difference.

Hopefully you can help, thanks!